The production of stable, aqueous polyurethane-polyurea dispersions is known and is described for example in German Offenlegungsschrift Nos. 1,495,745; 1,770,068; 2,555,534; 2,446,440 and 2,543,091; U.S. Pat. Nos. 3,479,310; 3,756,992; 4,108,814 and 4,094,842; and D. Dieterich et al, Angew. Chem. 82, 53 (1970).
In this connection, a distinction has to be drawn between processes involving the use of solvents and so-called solvent-free processes.
In the first case, high molecular weight polyurethanes are synthesized by polyaddition in organic solution, i.e. in homogeneous phase, before they are dispersed in water. This process gives polyurethane films of particularly high quality which may even be used for coating textiles.
The disadvantage of this process lies in the poor volume/time yield which results from the large quantity of solvent required and which, in addition, has to be rectified after distillation.
In addition, the use of the organic solvent increases the danger of fire and explosions in the production plants.
Of the solvent-free processes, two are particularly worth mentioning, namely the so-called melt dispersion process which is described in German Offenlegungsschrift No. 1,770,068 and in U.S. Pat. No. 3,756,992, and the process described in German Offenlegungsschrift No. 2,446,440 and in U.S. Pat. No. 4,108,814 in which a prepolymer containing sulphonate and NCO-groups is reacted with diamine during or after dispersion, the reaction being accompanied by chain extension.
Whereas the melt dispersion process gives crosslinked polyurethanes the properties of which are inadequate for many applications, for example the coating of textiles, the process described in German Offenlegungsschrift No. 2,446,440 gives partly inhomogeneous polyurethane urea particles which can in turn lead to films with inadequate mechanical strength and, in some cases, inadequate resistance to hydrolysis. The reason for the inhomogeneity of the polyurethane ureas lies in the procedure adopted in German Offenlegungsschrift No. 2,446,440 which does not guarantee uniform dispersion of the diamine in the prepolymer containing NCO-groups before chain extension.
Accordingly, the object of the present invention is to provide a solvent-free or low-solvent process which makes it possible for polyurethane (ureas)s with improved properties to be produced in the aqueous phase, the improvement being derived in particular from the fact that the high molecular weight solid dispersed in the liquid aqueous medium is obtained by polyaddition in homogeneous phase.
This object is achieved by the process according to the invention as described in detail hereinafter.